The present invention relates to the formation of optical fiber preforms by the solution doping of porous glass.
Optical fibers have been routinely fabricated on a commercial basis with losses less than 1 dB/km in at least part of the optical region of the spectrum, generally extending from 0.7 to 1.7 microns. The fibers comprise a core and a cladding, with the cladding having an index of refraction lower, at least in part, than that of an index of refraction associated with the core. Such low loss optical fibers are formed of glass comprising primarily silica, i.e. the glass composition comprises more than 50% silica.
Dopants which are used to make optical fibers include germania, an index raising dopant, which is the principal and most widely used dopant, as well as other minor dopants, such as phosphorus, and other index raising dopants, and fluorine and boron, index lowering dopants. Other dopants considered for use in optical fibers include Al, Zr, Nb, Ta, Ga, In, Sn, Sb, Bi, the 4f rare earths (atomic numbers 57-71), and the alkaline earths Be, Mg, Ca, Zn, Sr, Cd, and Ba. Of these, certain rare earth-doped optical fibers are of interest for a variety of applications including fiber lasers, attenuators and sensors.
Optical fibers are normally made by the oxidation of metal chlorides. Chlorides are conventionally used because they can be vaporized at relatively low temperatures and delivered to a hot zone where they are oxidized. By "hot zone" is meant that region of a glass preform forming apparatus where glass forming reactant vapors are oxidized; it can include, for example, a region within a burner flame or a heated region within a substrate tube. Vaporization techniques typically used for silicon tetrachloride and germanium tetrachloride include bubbling, direct vaporization and flash vaporization. Other chlorides that have been used commercially include boron and phosphorus chlorides which are also liquid or gaseous at room temperature. There are however several other metal chlorides that could be used in optical waveguides that are solids at room temperature and may or may not sublime rather than boil. These properties make it nearly impossible to deliver these materials with conventional systems.
Solution doping techniques have been employed for incorporating into glass preforms dopants which are not easily delivered to the reaction zone or which cannot be incorporated in adequate quantity when introduced into the glass during its formation in the hot zone. See, for example, U.S. Pat. No. 3,859,073 (Schultz) and the publication: J. E. Townsend et al. "Solution-Doping Technique for Fabrication of Rare-Earth-Doped Optical Fibers", Electronics Letters, 26 Mar. 1987, vol. 23, No. 7, pp. 329-331. The Schultz patent relates to preforms formed by the so-called "outside process" in which a porous preform is deposited on the outer surface of a cylindrical mandrel. The Townsend et al. publication relates to the so-called "inside process" whereby a porous coating can be built up on the inner surface of a substrate tube.
In accordance with the aforementioned Schultz patent one or more reactant compounds are delivered in vapor form to a burner; they react in the flame to form glass particles that are deposited to form a porous preform having a network of continuous open pores throughout its mass. The particles within the preform must adhere to one another to a sufficient extent that they will not separate and cause the preform to disintegrate when it is in contact with liquid. At the same time, a network of continuous pores is required for effective impregnation by a dopant solution. Thus, the particles cannot be so densely packed as to interfere with liquid entry into the preform during impregnation. The Schultz patent teaches that porosity should be about 75% for optimum processing and that bodies having porosities within the range of 60-90% can be useful. As a general rule, pore diameter is said to be within the range of 10.0 and 0.001 microns. The porous preform is cooled and then immersed in a solution containing a dopant, whereby at least a portion of the pores is filled with dopant material which deposits as a solid in the pores. The porous preform is dried and heat treated to consolidate it into a non-porous glassy body containing the dopant. If the resultant glass article is to form the core or central portion of an optical fiber, it is provided with cladding material and drawn into a fiber. The cladding can be added by inserting the doped glass article into a cladding glass tube or by depositing additional cladding glass particles on the outer surface of the doped glass article.
Solution doping techniques have commonly employed aqueous or alcoholic solutions of the dopant compounds. Perhaps porous coatings formed by the inside process can withstand the effects of such solutions since the porous region is supported by an outer silica wall. However, when optical fiber preforms have been formed by processes including immersing porous preforms in an aqueous or alcoholic solution, the preforms have often been rendered useless due to either disintegration during immersion in the solvent or cracking of the outer layers of the preforms during drying. Larger preforms, which are preferred for use in commercial operations, exhibit a greater tendency to fracture during immersion in water or alcohol. Such damage may be caused by stress resulting from hydrogen bonding of the solvent to the silica surface which is of extremely large area in porous optical fiber preforms.